To edit text files on a Plugcomputer, a text-mode text editor is needed which runs in your terminal. I am using vim (viimproved) as console text editor.
If vim is not installed on your Plugcomputer, install it from the repositories:
apt-get install vim
With vim, you can open text files easily with the following command:
Once a text file is opened, it shows it's content and you can navigate with the cursor and with your mouse wheel. You will recognize that pressing a key does not write the pressed key but instead might do some weird stuff. Press 'a' or 'i' and vim will get into edit mode (indicated at the bottom of the terminal with "-- INSERT --"). Now vim is ready to edit your text file. If you want to execute a vim command, the first thing to do is to press ESC. Then, every command is executed after a ':' and the specific command. The following list shows the most important commands:
|:q!||exit vim and discard changes|
|:wq||save and exit|
Take a look at the The Vim commands cheat sheet for more commands.